1. Field of the Invention
The present invention relates to the structure of a single-phase-structure actuator, and more particularly to a rotor structure of an actuator, which is easy to assemble, has well-balanced holding torque and detent torque, and is stable repetitive rotation operation.
2. Description of the Related Art
The inventors of the present application have already proposed actuator technology for an actuator of a single-phase structure claw pole-type structure, according to which a stopper mechanism is provided internally, and a rotor is made to perform repetitive rotation operations by energizing a coil in this kind of actuator, it is desirable to enlarge the rotation angle range while ensuring detent torque.
However, generally speaking, increasing detent torque causes a narrowing of the rotation angle range and therefore the detent torque must be reduced to increase the rotation angle range. That is, detent torque and rotation angle range are reciprocal in nature. This tendency is particularly notable when a sintered high-performance rare earth magnet (for example, an Nd magnet or Sm magnet), having magnetic anisotropy (for example, polar anisotropy), is used to make an actuator smaller.
As a technique applied to rotor magnet for ensuring detent torque and increasing rotation angle range, there is a method by which, when the rotor magnet is go two poles for example, a groove or cut is formed axially in the polar center portion of one of a pair of poles (for example N pole only) to increase the rotation angle range by losing the magnetic balance between the N pole and the S pole (Refer to FIG. 5). However, the drawback is that numerous processes are required to ensure the precision of the shape of the groove or cut portion (specifically, the central angle .alpha. of the groove or cut), making processing complicated. The reason for this is because, in the case of a sintered magnet, the circumference of the magnet is polished after sintering. If there is a groove or cut in the magnet circumference, not only is it impossible to ensure cylindricity, but it is also very difficult to maintain a stable groove or cut shape. Another drawback is that machine processing of the groove or cut raises increase in costs.
Therefore, in consideration of magnet processing, precision maintenance, and costs, it is desirable that the shape of the circumference of a magnet is a cylindrical shape without any groove or cut.